1. Field of the Invention
The present invention relates to an electric power steering apparatus for a vehicle. The present invention also relates to a power transmission gear suitable for use of the electric power steering apparatus.
2. Related Background Art
A steering apparatus for a vehicle has hitherto been classified into a manual type and a power assist type. The latter power assist type is designed to relieve the manual force by assisting the manual steering force with power taking some form. This power assist type steering apparatus is further classified into an electric type and a hydraulic type.
The former electric type is known as an electric power steering apparatus. Within this category, an electric power steering apparatus with a deceleration mechanism using a ball screw nut is disclosed in, e.g., Japanese Patent Application Post-Exam Publication No. 6-504. FIG. 19 illustrates a sectional configuration of this steering apparatus. Referring to FIG. 19, a first gear 21 is fixed to a rotary shaft of a motor 22. A second gear 32a formed integrally with a ball screw nut 32 meshes with the first gear 21. The ball screw nut 32 is rotatably supported by a housing 25 through bearings 28. A rack shaft 33 is internally fitted in the ball screw nut 32. The rack shaft 33 is formed with a helical groove 33b indirectly engaging through balls 26 with a helical groove 32b of the nut 32. A rack 33a is formed on a left sided portion, as viewed in FIG. 19, of the helical groove 33b of the rack shaft 33, and meshes with a pinion gear 36 to which a rotation of a handle 35 is transferred via a steering shaft.
The ball screw type electric power steering apparatus in the prior art adopts the deceleration mechanism constructing by combining the gears with each other as the first gear 21, the second gear 32a and the ball screw nut 32 are combined, and therefore large operating noises emit due to sliding on a contact portion of a metal and so on. Especially in the case of the deceleration mechanism using the gears composed of steels, there exists a problem in which butting noises occur due to gaps between the teeth, and a driver of the vehicle might feel the noises uncomfortable.